1. Field of the Disclosure
The present invention relates to a method and apparatus for reporting power headroom in a mobile communication system supporting carrier aggregation and, more particularly, to a method that determines maximum transmit power for each of multiple carriers and reports the determined maximum transmit power to a base station.
2. Description of the Related Art
In general, mobile communication systems have been developed to provide communication services while guaranteeing user mobility. Thanks to rapid technological advancement, mobile communication systems are capable of providing not only voice communication services but also high-speed data communication services.
Recently, the 3rd Generation Partnership Project (3GPP) has been working to standardize specifications for the Long Term Evolution (LTE) system as a next generation mobile communication system. The LTE system aims to realize high-speed packet based communication supporting a data rate of 100 Mbps exceeding existing data rates, and the specification thereof is near completion.
After completion of the release 8 specification, various technologies have been introduced to the LTE system to meet the ever increasing traffic demand. Carrier aggregation (CA) is a representative technology introduced to the release 10 LTE specification.
In existing communication, a single carrier is used in between a user equipment (UE) and a base station (ENB). When carrier aggregation is employed, a primary carrier and one or more secondary carriers may be used for communication between one UE and ENB, significantly increasing the data transfer rate by an amount corresponding to the number of secondary carriers. In other words, it may be considered that one cell is formed by a downlink carrier and an uplink carrier provided by the same ENB. In carrier aggregation, one UE may be considered as sending and receiving data through multiple cells in parallel. In this case, the maximum data rate of the UE may be increased in proportion to the number of aggregated carriers or cells.
In the LTE system, each carrier may be termed a component carrier (CC). A primary carrier may be termed a primary cell (PCell), and a secondary carrier may be termed a secondary cell (SCell).
Frequency resources are limited. As different countries may use different frequency resources or bands, it is not always possible to aggregate carriers belonging to the same frequency band only depending on circumstances. To solve this problem, the release 11 LTE specification provides enhanced carrier aggregation so that carriers belonging to different frequency bands can be aggregated together.
The fact that a UE receives data through a downlink carrier and transmits data through an uplink carrier may correspond in meaning to a case in which the UE sends and receives data using control and data channels provided by a cell corresponding to the center frequencies and frequency bands characterizing the carriers.
For the purpose of the description of the present invention, the terms used in general LTE system documents apply. For more details, TS 36.331 and TS 36.321 (2011 December) may be referred to.
Meanwhile, for uplink scheduling, a UE reports various scheduling information (for example, buffer status report and power headroom (PH) report) to the corresponding ENB. The ENB may allocate a suitable amount of uplink transmission resources to the UE in consideration of the buffer status and power headroom of the UE.
When carrier aggregation described above is employed, a UE may perform uplink transmission at more than one serving cell. As uplink transmission at multiple cells and uplink transmission at one cell may have different attributes, power headroom reporting (PHR) at a single cell may not be directly applicable.